This invention relates to characterizing a laser module that contains an internal etalon. In particular, the invention relates to using an external etalon as a wavelength reference to characterize a wavelength selectable laser that contains an internal etalon.
Recently, wavelength selectable lasers have been designed to include an internal etalon that is used as a wavelength reference for laser optical output. It is known in the art to use a Fabry-Perot etalon as the internal etalon. A Fabry-Perot etalon is an etalon in which the end faces of a solid, transparent body are used as the etalon reflecting surfaces. Coatings may be applied to one or both surfaces to enhance performance. A known disadvantage of Fabry-Perot etalons is that changes in temperature cause changes in the thickness and index of refraction of the material making up the solid transparent body of the etalon. Thus, it is known in the art that an internal etalon used as a wavelength reference in wavelength selectable laser products is highly temperature dependent with respect to its periodic output response as a function of wavelength. For this reason, all wavelength selectable products containing etalons must be fully characterized to provide look-up information required to control and lock the wavelength of the product. Typically, this characterization is very time consuming and therefore very costly.
The prior art discloses methods that make use of wavemeters or xe2x80x9cscanning typexe2x80x9d optical spectrum analyzers to characterize wavelength selectable laser products. These instruments create a look-up table by comparing their measured value to the internal etalon response. The look-up tables define the channels upon which the wavelength of the product will be locked. However, using scanning type wavemeters or optical spectrum analyzers is time consuming, on a per unit basis, and cannot be made practical for use in high volume manufacturing.
For example, using an optical spectrum analyzer to characterize a wavelength selectable laser product can take as long as one second per data point to accumulate the requisite number of data points to adequately characterize the product, which is typically 500 data points. The amount of time it takes to measure one data point becomes significant when a large amount of data points are required to fully characterized a single device.
A need exists in the laser products industry for high speed testing and characterization of wavelength selectable laser products to maximize product throughput and, thus, reduce costs in a manufacturing environment. The present invention addresses this need by providing a method and apparatus for virtually instantaneous characterization of wavelength selectable laser products by using an external etalon as an absolute wavelength reference. A structure of the present invention may also be adapted to decrease the testing time for making time-resolved spectroscopy measurements, such as wavelength chirp, which is a typical measurement required for designing and manufacturing most distributed feedback lasers, distributed Bragg-Reflector lasers, and electroabsorption modulated laser sources.
The invention relates to laser module characterization apparatus and a method therefor. The apparatus is preferably controlled by a computer and includes a laser module, an internal etalon transducer module, an external etalon, and an external etalon transducer. The laser module has a laser source substantially simultaneously emitting a pair of substantially identical beams of light, and an internal etalon adapted to receive a first of the pair of beams of light and to permit a portion of the first beam to pass through the internal etalon. The internal etalon transducer is connected to the computer and is responsive to the portion of the first beam. The external etalon is adapted to receive a second of the pair of beams of light and to permit a portion of the second beam to pass through the external etalon. The external etalon transducer is connected to the computer and is responsive to the portion of the second beam.
According to a method of the invention, the laser module is characterized by (1) energizing a laser source of the laser module at a first wavelength; (2) substantially concurrently measuring a response of the internal etalon and a response of an external etalon to the first wavelength of the laser source; (3) substantially concurrently storing the internal etalon response and the external etalon response in a memory; (4) comparing the internal etalon response to the external etalon response at the first wavelength; and (5) generating a characteristic of the laser module according to the comparison.